we used the landscaping timbers that are about 5 inch diameter that is planed top & bottom, makes it about 3.5 inches tall, use 4-4.5 inch deck screws while overlapping/alternating ends, the picture is 3 high but I added one more layer to be 4 high (~12-13 inches deep). We bought 9 full length (8') to make a 4'x8' x 3 high bed (but again, added one more layer the following year). Pick up at Home Depot and get enough cut in half there (free) for the 4' cross pieces.
We typically can plant 4-5 tomato plants and 3-4 squash in one 4x8 bed. We had 6 tomato and 3 squash this year but it was pretty crowded. I took a small trailer to a garden center to buy bulk garden soil, got about 1/2 yard worth, backed trailer up to bed and shoveled in to almost the top. once I got all plants planted, I covered the top with hardwood mulch to keep weeds down. This worked very well and the following year, I took a spade to it and mixed the composting mulch into the soil along with a couple bags heavily composted soil.
the second picture is one I did for my daughter down in Katy but she has soil underneath, so the 3 high worked well. She planted 4 tomato down one side, 2 yellow squash, several strawberry, several jalapeno with a small row of okra. it was pretty well laid out for maximum usage. you can tell a lot more in the second picture what I was talking about with the alternating/overlapping the ends of the timbers.